1. Field of the Invention
The present invention relates to an automatic digital frequency control circuit adapted for use in a digital video signal processing apparatus and, more specifically, to an automatic digital frequency control circuit capable of generating a frequency signal which is synchronized to a horizontal sync signal without oscillating a separate comparison frequency by using a phase locked loop circuit.
The instant application is based on Korean Patent Application No. 94-4803, which is incorporated herein by reference for all purposes.
2. Description of the Related Art
A video cassette recorder generally converts a carrier chrominance signal to a low-band frequency and records the converted signal on tape. A low-band carrier frequency in which the phase is synchronized to a horizontal sync signal is required to convert the carrier chrominance signal to the low-band frequency. The low-band carrier frequency is approximately 40f.sub.H (=629KHz) for VHS (Video Home System) recording, and 47.25f.sub.H (=743KHz) for 8 mm recording, respectively, as compared with a horizontal sync signal frequency f.sub.H. Thus, an automatic frequency control circuit needs to precisely generate a low-band carrier frequency of 40 cycles (VHS standard) or 47.25 cycles (8 mm system) every scanning line.
As shown in FIG. 1, a circuit for generating a low-band carrier frequency, which is synchronized to a horizontal sync signal, in an analog video signal processing circuit includes a phase locked loop circuit. In FIG. 1, a horizontal sync separator 10 separates a horizontal sync signal HD from an input luminance signal and supplies the separated horizontal sync signal HD to a phase comparator 20. Phase comparator 20 generates a control direct-current (DC) voltage and supplies the control DC voltage to a voltage controlled oscillator (VCO) 30, which generates a signal F.sub.1 and supplies signal F.sub.1 to a first frequency divider 40. First frequency divider 40 frequency-divides signal F.sub.1 by a factor of four (4), to generate a signal F.sub.2 and a low-band carrier frequency signal. A second frequency divider 50 receives signal F.sub.2 and frequency-divides received signal F.sub.2 by 40, to generate a signal F.sub.3. Phase comparator 20 generates the control DC voltage based on a phase comparison result between horizontal sync signal HD and signal F.sub.3.
The operation of the apparatus depicted in FIG. 1 will now be described below for the exemplary case wherein the circuit is employed in a VHS apparatus.
When a luminance signal of a color video signal is input to horizontal sync separator 10, a horizontal sync signal HD is separated from the luminance signal and output to phase comparator 20. Phase comparator 20 compares the phase of signal F.sub.3, applied from second frequency divider 50, with that of horizontal sync signal HD and generates the control DC voltage corresponding to the phase difference between the respective signals. The control DC voltage is supplied to VCO 30 and is used for controlling the oscillating frequency. During initial operation, VCO 30 generates signal F.sub.1 having a frequency of 160f.sub.H ; first frequency divider 40 generates signal F.sub.2 having a frequency of 40f.sub.H ; and second frequency divider 50 generates signal F.sub.3 having a frequency of f.sub.H. It will be noted that, if control DC voltage signal is generated according to the phase difference of signal F.sub.3 and horizontal sync signal HD, the frequency of signal F.sub.1 generated by VCO 30 is varied. Accordingly, the frequencies of signals F.sub.2 and F.sub.3 are also varied. If VCO 30 generates signal F.sub.1 in which the frequency is varied according to the magnitude of control DC voltage, first frequency divider 40 receives signal F.sub.1 and generates signal F.sub.2 which is synchronized to horizontal sync signal HD. Then, second frequency divider 50 receives signal F.sub.2 and generates signal F.sub.3 which is synchronized to horizontal sync signal HD. Signal F.sub.2 generated in first frequency divider 40 is supplied to a carrier converter (not shown) and is used as a low-band carrier signal in which the phase is synchronized to horizontal sync signal HD.
In the analog circuit as described above, VCO 30 is used to generate a high frequency signal and the oscillated signal is down-converted to a lower frequency signal in order to obtain a desired frequency signal. However, the above-described operating method cannot be adapted to a digital circuit, since the VCO 30 cannot be used. Additionally, it is difficult to generate a high frequency signal using a digital circuit. Since the signal processing method for many video cassette recorders have been recently changed from analog to digital processing, if a low-band carrier frequency generating algorithm which is used in an existing analog system, is adapted to a digital circuit for recording a chrominance signal as it is, a volume of the circuitry becomes prohibitively large.